The present invention broadly relates to ground fault protection of parallel connected generators of an electrical network, and, more specifically, is concerned with a new and improved method of, and apparatus for, detecting ground shorts in an electrical network containing generators connected in parallel, wherein in the presence of a ground short at the generator a ground or earth current flows between the fault location and a ground location disposed at a suitable place or site, and the electrical network at the ground location is grounded by means of a ground transformer and a ground resistance or resistor.
In order to selectively protect generators connected directly with a busbar against ground shorts, present day technology employs a null current measurement at the generator output side. For explanatory purposes reference is made to a prior art ground fault protection system as shown in FIG. 1, wherein in the presence of a ground short at one of the generators a ground or earth current Iw flows between the fault location and a ground location E disposed at a suitable place or site. At the ground location E the electrical network 1 is grounded with the aid of a ground transformer 120 through a ground resistor Ra and Rb respectively. The current Iw is a purely active current. Advantageously, it is measured at location A by means of a directional relay RR. By means of this directional relay RR there are likewise filtered out the capacitive currents flowing at location A. The site of the ground resistor Ra or Rb is dependent upon the specific grounding technique. With all of the state-of-the-art methods the current Iw however cannot be chosen to be smaller than the fault currents If of the null current converter. The present day conventional ground active currents Iw require complicated ground transformers and other ground elements, such as ground protective devices and ground resistances.
A further drawback of all of these prior art techniques resides in the fact that the ground short-active current Iw, and therefore, also the measurement time, decreases when the ground short has shifted towards the star point of the electrical machine.